The ability to enter animal cells is a property common to many pathogenic microorganisms. The entry mechanism of these pathogens has been the subject of much research in recent years and several different species and genera have proven to be useful models. The simplest system identified thus far is that of Y. pseudo tuberculosis and Y. enterocolitica, where individual invasion factors (either invasin or Ail) are sufficient to confer the invasive phenotype to non-invasive E. coli. In contrast, entry (invasion) by Salmonella is more complex involving at least two different components to act in concert. My laboratory has been using Y. enterocolitica and S. enteritidis as model systems to study entry of bacteria into eukaryotic cells. The long-term goals of our work are to understand the bacteria-host interaction at the molecular level, and to determine what the role of entry is in the pathogenesis of infections. For S. enteritidis we have been characterizing entry defective mutants. For Y. enterocolitica our work has focused on two genes, inv and ail, which encode proteins that can promote entry. The experiments described in this proposal are aimed at further characterizing these genes, their products, and their role in entry and pathogenesis. Specifically we propose to address the following questions: 1) How many S. enteritidis invasion genes are there and what are their products? 2) What is the effect of mutations affecting entry on the virulence of S. enteritidis? 3) What is the effect on virulence of mutations in inv and/or ail of Y. enterocolitica? 4) How does Ail promote invasion? An understanding of the process of bacterial invasion, should make it possible to devise better methods of treatment and prevention in the future.